Pathogenic Analysis of HP-PRRSV in Experimentally Infected 4-month-old Laiwu Black Pigs

The objective of this study was to study the pathogenic characteristics of highly pathogenic of porcine reproductive and respiratory syndrome virus(HP-PRRSV) on local pig breeds.Fore-month-old healthy Laiwu black pigs were inoculated nasally with HP-PRRSV.The necrops of pigs were conducted on different days post inoculation.The antibody titers of peripheral blood serum were tested by ELISA test kits(IDEXX).Tissues were collected for preparing tissues sections.Pathological changes of different tissues were observed through HE staining.Immunohistochemistry(IHC) protocol were developed to detect distribution of HP-PRRSV antigens in tissue samples.The results showed that slight symptoms could be seen 2 days post-infection,lasting two days before disappearing and no pigs died.Antibodies can be detected 5 days post-infection(1/7),and reached the peak 14 days post-infection.Histopathological changes showed that interstitial pneumonia,slight viral encephalitis and particles degeneration of essence organs occurred in pigs 3 days post inoculation;typical interstitial pneumonia,necrosis of the lymphoid organs,eosinophilic cells infiltration in intestines were found and parenchymal organs appeared vacuolar degeneration 7 days post inoculation.From 2 weeks to 4 weeks post inoculation,typical interstitial pneumonia were found in lung,and the viral encephalitis becoming seriously,necrosis in adrenal,inflammatory cells infiltrating in pancreas were slight,serious congestion and hemorrhage were found in thyroid and parenchymal organs appear vacuolar degeneration acutely.PRRSV antigen-positive signals could be seen in lymphoid tissue(lymph nodes,spleen,tonsils),trachea,heart,liver,lung,kidney,stomach,duodenum,jejunum,ileum,thyroid,adrenal gland,submandibular gland,uterus,brain and cerebellum;Cecum,colon,rectum,bladder,pancreas,fallopian tubes and ovaries were not seen positive signals.Positive signals were mainly located in the cytoplasm of infected cells,occasionally in the nucleus.This experiment confirmed that infection of 4-month-old Laiwu black pigs with HP-PRRSV could cause a wide range of tissue tropism and extensive tissue damage,while clinical symptoms are mild;no mortality;4-month-old Laiwu black pigs have a strong resistance to HP-PRRSV.The results provide a theoretical basis to study the differences in the pathogenicity of HP-PRRSV between our local species of pigs and the foreign ones.